1. Field of the Invention
This invention relates to electrodeposition of metals in a finely divided state and more particularly to electrodeposition of catalytic metals for fuel cell electrodes.
2. Brief Description of the Prior Art
Among the several types of fuel cells under development to provide efficient sources of electrical power with reduced pollution, cells using gas diffusion electrodes (GDEs) with proton exchange membranes as the electrolyte (proton exchange membrane fuel cells, PEMFCs) are seen as having a number of advantages. Such fuel cells avoid the problems of handling liquid fuels and electrolytes because they use gaseous reactants and a solid electrolyte that allows the transfer of protons between electrodes. They have been found to be reliable, efficient and convenient sources of power. However, they have proved to be very expensive in terms of cost per kilowatt of power delivered. As a consequence their practical application has been limited to specialized applications that can justify their considerable expense, e.g., in aerospace applications. If such fuel cells are to achieve wider application, for example as sources of power for automotive propulsion or stationary power plants, the cost in terms of dollars per delivered kilowatt will have to be significantly reduced.
A major factor in the cost of PEMFCs is the expense of the electrodes. The cost of the electrodes is determined by a number of factors, principally the expense of the precious metal catalysts, which are needed for practical efficiency, and the cost of fabricating the electrodes, which is typically conducted by means of a batch process. Furthermore, the cost of the fuel cell system is also greatly affected by the electrochemical performance of the electrodes which determines the power density of the fuel cell, i.e., the power produced per unit area, e.g., kilowatts per square centimeter. The combination of power density, catalyst loading and system fabrication costs determines the ultimate cost per kilowatt of the complete fuel cell system.
Conventional fuel cell electrodes have used unsupported platinum black, having a surface area of about 28 m.sup.2 /g with a particle size of about 10 nanometers, at a catalyst loading of about 4 mg/cm.sup.2 of electrode area. It is estimated that the amount of precious metal will have to be reduced substantially below 1 mg/cm.sup.2 if PEMFCs are to become a widely used source of electric power.
It has been recognized that the amount of precious metal catalyst can be reduced if the metal is present in a more finely divided form. Consequently, electrodes using platinum supported on a granular support, e.g., carbon particles, have been used. Such supported platinum catalysts, prepared by chemical precipitation of the metal onto the granular support, typically have surface areas of about 120 m.sup.2 /g, with a particle size of about 2-2.5 nanometers, and a catalyst loading of about 0.5 mg/cm.sup.2. Although these electrodes use less of the costly platinum catalyst, the power density obtained using such electrodes has been less than satisfactory. Accordingly, the cost of such a fuel cell system is still too high. It is believed that the relatively poor performance, i.e., low power density, is caused by ineffective utilization of the catalyst because a substantial fraction of the platinum is not accessible to the reagents.
A method for depositing precious metal catalyst in finely divided form in a gas diffusion electrode is disclosed in U.S. Pat. No. 5,084,144, to Vilambi-Reddy et al., the entire disclosure of which is incorporated herein by reference. According to the method of U.S. Pat. No. 5,084,144, fine particles of a catalytic metal are deposited electrolytically onto an uncatalyzed layer of carbon particles, bonded with a fluorocarbon resin and impregnated with the proton exchange resin, by contacting the face of the electrode with a plating bath and using pulsed direct current. The gas diffusion electrodes prepared by the process of U.S. Pat. No. 5,084,144 contained about 0.05 mg/cm.sup.2 of platinum as particles of about 3.5 nanometers in diameter having a surface area of about 80 m.sup.2 /g. Such electrodes functioned about as well as the electrodes using supported platinum with a loading of 0.5 mg/cm.sup.2 of platinum. It is believed that these electrodes achieved their improved mass activity, i.e., current per weight of platinum, because the electrolytic process deposits the catalyst particles only at regions with both electronic and ionic accessibility. Such locations are expected to be accessible to the protons and electrons required for the fuel cell reactions. However, such improved mass activity does not compensate for the low catalyst loading provided by the process of U.S. Pat. No. 5,084,144. Consequently, the power density of such electrodes is still insufficient to permit the wide use of PEMFCs as sources of electric power.
Accordingly, a need has continued to exist for a method of depositing catalytic metals in gas diffusion electrodes in amounts greater than hitherto achieved, while retaining the small particle size and electronic and ionic accessibility that provides high mass activity.